1. Field of the Invention
The invention relates to an optical scanning device for scanning an information plane of a disc-shaped optical recording medium having an underlying surface extending parallel to the information plane. Such scanning device comprises a radiation source for supplying a scanning beam, and an optical scanning unit having a lens system for focusing the scanning beam to a scanning spot on the information plane. The lens system has an optical axis and comprises a first lens unit with a first lens having an entrance side for the scanning beam and an exit side situated opposite thereto, and a second lens unit with a second lens situated opposite the exit side of the first lens. The scanning unit also comprises drive means for displacing the first lens along the optical axis, and adjusting means for adjusting the positioning of the second lens with respect to the underlying surface of the recording medium.
2. Description of Related Art
A scanning device of this type is known from International Symposium on optical memory and optical data storage, conference edition Jul. 8-12, 1996 (ISOM 96); OFA 2-1/345--OFA 2-3/347; A. 0.8 Numerical Aperture Two Element Objective Lens for the Optical Disc; Kenji Yamamoto et al. The known device is intended for scanning a magneto-optical disc and is provided with a first holder having an objective lens and a transparent plate, and a second holder having a solid immersion lens (SIL). The magneto-optical disc has an information layer and a transparent layer, which information layer is read via the transparent layer. For focusing and tracking, a focusing actuator and a tracking actuator are provided for driving the first holder. A SIL actuator is present for displacing the second holder with the solid immersion lens in the focal direction in dependence upon a change of capacitance of a capacitor formed between the two holders. This has for its object to inhibit spherical aberration due to thickness deviations of the transparent layer. In this connection, reference is also made to JP-A 08212579, published Aug. 20, 1996. In addition to thickness variations, an optical disc located on a rotating table may be curved, for example bent through due to the fact that the disc is only supported in its center during scanning. Consequently, the optical disc has a certain tilt with respect to the scanning beam, which results in coma and astigmatism during scanning. Such optical aberrations are not prevented by the known measures.